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Omnichain Contracts
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Bitcoin Frontend

Bitcoin Frontend

In this tutorial you will learn how to build a simple frontend application that let's you call omnichain contract on ZetaChain from Bitcoin using a browser wallet extension.

For the purposes of this tutorial we will be using the XDEFI wallet.

Prerequisites​

Create a new project

Initialize a new project:

npm init -y

Install a simple HTTP server:

yarn add --dev http-server

HTML Page with a Simple Form​

Create a page with a simple form:

index.html
<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>Bitcoin</title>
  </head>
  <body>
    <input type="number" id="amount" placeholder="Amount in tBTC" />
    <input type="text" id="contract" placeholder="Omnichain contract address" />
    <input type="text" id="params" placeholder="Contract call parameters" />
    <button onclick="sendTransaction()">Send transaction</button>
    <script src="./script.js"></script>
  </body>
</html>

This HTML page contains a simple form with three inputs:

  • Amount - the amount of tBTC to send
  • Contract - the address of the omnichain contract to call
  • Params - the parameters to pass to the contract call
  • Send transaction - a button that will call the sendTransaction function

Use XDEFI to send a transaction​

Implement the sendTransaction function:

script.js
async function sendTransaction() {
  const bitcoinTSSAddress = "tb1qy9pqmk2pd9sv63g27jt8r657wy0d9ueeh0nqur";
  const wallet = window?.xfi;
  if (wallet === undefined) return alert("XDEFI wallet not found");

  const account = (await wallet?.bitcoin?.getAccounts())?.[0];
  if (account === undefined) return alert("No account found");

  const contract = document.getElementById("contract").value.replace(/^0x/, "");
  const params = document.getElementById("params").value.replace(/^0x/, "");

  let memo = ""; // Default empty memo
  if (contract.length === 40) {
    // Contract looks like a valid address, sending an encoded memo
    memo = `hex::${contract}${params}`;
  }

  const amount = parseFloat(document.getElementById("amount").value) * 1e8;
  if (isNaN(amount)) return alert("Amount must be a number");

  window.xfi.bitcoin.request(
    {
      method: "transfer",
      params: [
        {
          feeRate: 10,
          from: account,
          recipient: bitcoinTSSAddress,
          amount: {
            amount,
            decimals: 8,
          },
          memo,
        },
      ],
    },
    (error, result) => {
      console.log(error, result);
    }
  );
}

To call an omnichain contract from Bitcoin you need to send a token transfer transaction to the ZetaChain's TSS address on Bitcoin with a memo that conforms to the required format.

The memo is composed of two parts:

  • The first part is the contract address encoded as hex string (without the 0x prefix). This is the address of the omnichain contract you want to call.
  • The second part is the parameters encoded as hex string (without the 0x prefix). This is the parameters you want to pass to the contract call.

For XDEFI to properly encode the memo, you need to prefix it with hex::.

For example, if the omnichain contract address is 0xc79E6DC99C5928C5b08ae7a0f79412521996938e and it expects to recieve one argument that happens to be an address 0x3724C896Cdc958611873D81547A98565c8cb849d, the memo passed to XDEFI would look like:

memo: "hex::c79E6DC99C5928C5b08ae7a0f79412521996938e3724C896Cdc958611873D81547A98565c8cb849d"

XDEFI injects itself into the window object, so we can access it from the window.xfi property. Use the getAccounts method to get the list of accounts and use the first one.

The list of TSS address is available in the docs, for the purposes of this tutorial we will use a hard-coded value.

Get the contract address and parameters from the form and ensure that the 0x prefix is removed.

If the contract address is 40 characters long, it is a valid address and we can use it as the first part of the memo. If it is not 40 characters long, we will use an empty string as the memo.

  • If the memo is valid and contains a contract address, ZetaChain will deposit tBTC as ZRC-20 to the contract and call the contract with the provided parameters.
  • If the memo is empty, ZetaChain will deposit tBTC as ZRC-20 to the user's address on ZetaChain.

The amount is passed in tBTC, so we need to multiply it by 1e8 to convert it to satoshis.

Finally, call the request method on the window.xfi.bitcoin object to send the transaction.

Now that we have the from address (your Bitcoin address), recipient address (ZetaChain's TSS address), amount and a properly formatted memo, call window.xfi.bitcoin.request to create and broadcast a transaction.

Learn more about how to use XDEFI programmatically in the XDEFI docs.

Send a Transaction​

Start the HTTP server:

npx http-server

Open the page in your browser and fill in the form. You can test functionality with your own contract address or follow one of the provided tutorials, for example, the Staking tutorial to deploy a contract that you can call from Bitcoin.

Fill out the form and click the "Send transaction" button. You will be prompted to confirm the transaction in XDEFI. If you filled out the "contract" field, make sure that the memo in the prompt is prefixed with hex::.

Once the transaction is confirmed, you will see the transaction hash in the console. You can use a Bitcoin block explorer like Blockstream to check the transaction status.

Once the transaction is confirmed on Bitcoin, you can check the transaction status on ZetaChain using the cctx command from the Hardhat template.